The invention relates to an arrangement comprising a shaft and a part having a hub mounted on the shaft and comprising at least one tubular portion which is coaxial with the shaft and is constructed to be elastically deformable in radial directions, which tubular portion is acted upon by at least one releasible clamping member in order to be clamped onto the shaft, which clamping member can be mounted on said tubular member and surrounds it in a ring-like manner and comprises three inwardly directed clamping zones which are equispaced at angles of 120.degree. from one another and are each movable in a radial direction.
The invention further relates to a clamping member for such an arrangement, which clamping member is of a ring-like construction in relation to an axis of symmetry and which comprises three inwardly directed clamping zones which are equispaced at angles of 120.degree. from one another and are each movable in a radial direction.
The invention further relates to an expansion device for moving the application zones of such a clamping member of such an arrangement.
An arrangement of the type defined in the opening paragraph is known, for example from AT 365.808 B, which is herewith incorporated by reference. This known arrangement concerns a recording and reproducing apparatus for picture and sound signals, which can be recorded on and reproduced from a magnetic tape. For this purpose the apparatus comprises a drum-shaped scanning device around which the magnetic tape is partly wrapped in a helical path, which scanning device has a drum-shaped pan which is clamped onto a rotatable shaft by means of a clamping member and which forms a head support carrying the magnetic heads for scanning the magnetic tape and for recording and reproducing picture signals. The clamping member is formed by a flat ring which is substantially non-deformable in radial directions and from which three clamping tongues project inwardly towards a tubular portion of a hub of the head support, the clamping tongues being inclined relative to the plane of the flat ring and having free ends, forming clamping surfaces, which bear on a step of the tubular portion. During the manufacture of this clamping member an additional operation is required to provide the inclination of the clamping tongues, which is a disadvantage. Moreover, the inclination of the clamping tongues is tolerance dependent to a comparatively large extent, so that the clamping forces which can be applied by means of such clamping member exhibit a comparatively large spread and, consequently, no precise and reproducible clamping is possible. For moving the free ends of the three clamping tongues in radial directions relative to the tubular projection, in order to clamp the tubular projection onto the shaft, the known arrangement comprises an axially adjustable screwthreaded ring which acts on the ring forming the clamping member via a compression ring, an axial displacement of the screwthreaded ring, the compression ring and the clamping-member ring causing the inclination of the three clamping tongues to be changed, thereby moving the free ends of the three clamping tongues in radial directions. When the tubular portion in this known arrangement is clamped onto the shaft the head support should be retained both axially and tangentially relative to the shaft, so that during clamping the head support is subjected to external forces both in the axial and in the tangential direction. These external forces make it difficult to achieve a trouble-free and exact clamping with a minimal axial and a minimal radial runout. Moreover, this known construction leads to undefined frictional forces in the area between the screwthreaded ring, the compression ring and the clamping-member ring and in the area between the free ends of the clamping tongues and the step of the tubular portion, which forces are adverse to the obtainment of accurately defined, mutually equal and reproducible clamping forces for clamping the tubular portion onto the shaft. In addition, the inclination of the three clamping tongues in the prior-art construction gives rise to axial force components between the clamping tongues and the tubular portion, which in the long run may result in an undesirable axial displacement of the head support along the shaft. Another disadvantage of the prior-art construction is that a comparatively large number of pans are required for clamping the tubular portion onto the shaft and the clamping process is comparatively difficult to mechanize.